The present invention relates to the recovery of tar from raw coking gas.
More particularly, the invention relates to an improved method of recovering tar from the condensate of raw coking gas which is obtained by coking of preheated coal.
When coal is subjected to coking, a raw coking gas is obtained which contains, inter alia, hydrocarbons, inert gas components, water vapor and very fine solid particles. The gas, usually at a temperature of about 700.degree.-800.degree. C. and coming from the coking ovens, is collected in a raw gas main in which it is cooled by spraying cooling water directly into the main. This results in partial condensation and in a subsequently arranged tar separator the tar and solid components are separated out from the cooling water. The cooling water is constantly recirculated and losses are replenished by the addition of fresh water; the condensation stage has its own cooling water circuit.
The effectiveness of the tar separator is closely tied to the temperature of the condensate from which the tar and solid components are to be removed. The minimum acceptable operating temperature of the tar separator is about 75.degree. C.; significant downward deviation, to e.g. 72.degree. C., results in unsatisfactory operating results, which is to say that the water content in the separated tar will not be sufficiently low.
The customary practice heretofore has been to pump the separated water from the tar separator, or from the water collector of the same, directly to the raw gas main where it is in part sprayed into the main and in part into the riser-pipe elbows connecting the main with the riser pipes of the coke ovens, which is to say into the hot raw gas stream. As mentioned before, the raw gas has a temperature of about 700.degree.-800.degree. C.; due to its heat exchange with and the partial evaporation of the cooling water, the raw gas undergoes sudden, shock-like cooling to temperatures of about 100.degree. C. and condensate forms in the raw gas main. The raw gas main is connected to a suction conduit via which the condensate is evacuated from it, by running along the bottom wall of the main and of the suction conduit. The part of the cooling water which is admitted directly into the main aids in this evacuation, since it forms at the bottom wall a liquid stream which facilitates the condensate outflow. This stream, composed of cooling water and condensate, is directed from the suction conduit to the tar separator.
The raw gas cooled in the main is forwarded to the raw gas cooler, which is a condensation stage cooled by appropriate heat exchangers, and from there is passed on to further processing stations. Condensate forming in the raw gas cooler is collected in a receptacle where it has a temperature of about 30.degree. C.; thereafter it also is passed on to the tar separator. According to a variant of this prior-art method the condensate formed in the raw gas cooler and the condensate formed in the raw gas main and elsewhere, may first be recirculated into the raw gas main and only thereafter passed to the tar separator.
Under either of these prior-art operating conditions the overall heat balance of the total system causes an operating temperature of about 80.degree. C. to develop in the tar separator if--and only if--the coal being coked has a moisture content on the order of about 10%.
Coal comes in a variety of different qualities; related to coke-making this means that some types of coke have much better coking ability than others. As with most resources, there has in the past been a tendency to use up the better-quality coals in preferences to the poorer-quality ones. However, coal is a non-renewable resource and in many areas of the world the better-quality coals--including coking coals--are becoming increasingly scarce. This has led to intensive research, as a result of which it has been shown that the coking abilities of poor-quality coal can be substantially improved if the coal is preheated prior to coking. This, in turn, leads to a reduction of its moisture content, to the point where moisture is either absent or present only in very small amounts. When such preheated coal is coked, very little coal-originated water vapor is available for condensation in the main and suction conduit and the thermal energy which is supplied by the generous vapor development in the coking of moist (e.g. about 10% moisture) coal is lacking. This thermal energy consequently cannot be supplied to the tar separator, so that in the coking of preheated coal--all other operating conditions being unchanged--the tar separator operates at a temperature of only about 72.degree. C. Whereas temperatures higher than 75.degree. C. are beneficial (best results are obtained at 80.degree. C.), temperatures significantly lower than 75.degree. C. (such as 72.degree. C.) result in unacceptable operation. Moreover, the lack of adequate moisture causes other problems, such as a substantial concentration of chlorine compounds and other aggressive substances in the cooling water; this requires the constant addition of fresh water to the cooling water circuit to prevent these concentrations from reaching unacceptable levels. Evidently, the constant addition of fresh water further reduces the content of thermal energy in the cooling water circuit.
Yet, for the reasons explained earlier, the use of preheated coal is becoming increasingly important in the industry. Attempts have therefore been made to increase the tar separator temperature--recovery of tar as a coking by-product is, after all, an extremely important function--by supplying the requisite additional heat and moisture via the admission of hot steam from an external source, for example by admitting externally produced steam directly into the main. This is effective, but it requires the constant production and admission of external steam and results in a drastic reduction of operating economy for the entire process. It goes without saying, therefore, that this solution is disadvantageous, from the point of view of economy as well as from the point of view of energy efficiency.